Thyroid hormone-dependent development in Xenopus laevis: a sensitive screen of thyroid hormone signaling disruption by municipal wastewater treatment plant effluent.

Because thyroid hormones (THs) are conserved modulators of development and physiology, identification of compounds adversely affecting TH signaling is critical to human and wildlife health. Anurans are an established model for studying disruption of TH signaling because metamorphosis is dependent upon the thyroid system. In order to strengthen this model and identify new gene transcript biomarkers for TH disruption, we performed DNA microarray analysis of Xenopus laevis tadpole tail transcriptomes following treatment with triiodothyronine (T(3)). Comparison of these results with previous studies in frogs and mammals identified 36 gene transcripts that were TH-sensitive across clades. We then tested molecular biomarkers for sensitivity to disruption by exposure to wastewater effluent (WWE). X. laevis tadpoles, exposed to WWE from embryo through metamorphosis, exhibited an increased developmental rate compared to controls. Cultured tadpole tails showed dramatic increases in levels of four TH-sensitive gene transcripts (thyroid hormone receptor ß (TRß), deiodinase type II (DIO2), and corticotropin releasing hormone binding protein (CRHBP), fibroblast activation protein α (FAPα)) when exposed to T(3) and WWE extracts. TRß, DIO2, and CRHBP were identified as TH sensitive in other studies, while FAPα mRNA transcripts were highly TH sensitive in our array. The results validate the array and demonstrate TH-disrupting activity by WWE. Our findings demonstrate the usefulness of cross-clade analysis for identification of gene transcripts that provide sensitivity to endocrine disruption. Further, the results suggest that development is disrupted by exposure to complex mixes of compounds found in WWE possibly through interference with TH signaling.

A common environmental contaminant affects sexual behavior in the clawed frog, Xenopus tropicalis.

Behavior can be a sensitive measure of endocrine disruption from exposure to environmental contaminants. Xenopus tropicalis has become a developmental model system for evaluation of endocrine disrupting compounds because of its relatively rapid development and its sequenced, diploid genome. We used X. tropicalis as a model for endocrine disruption of sexual behavior. We injected frogs intraperitoneally (IP) with a gonadotropin hormone-releasing hormone agonist at 0.31 µg/50 µL or vehicle control solution and determined behavioral outcomes. Next, we used GnRH-induced sexual behavior to determine the effects of a 30-day exposure to aqueous estradiol (E2) at 10(-8) M or a common pollutant, 4-tert octylphenol (OP), at the environmentally relevant concentrations of 10(-7) M OP or 10(-8) M OP. The GnRH-agonist treatment had no effect on female behaviors. In males, GnRH-agonist treatment increased approaches, touches, amplexus, and a sum total of all sexual behaviors (total sexual behavior score). Exposure to E2 or any dose of OP had no effect on female behaviors. In males, E2 and 10(-7) M OP increased incidence of arm waving (a potential pheromone releasing behavior), and E2, and both doses of OP increased calling behavior compared to an unexposed control group. More males in all the exposure groups expressed sexual behavior than in the control group. This study demonstrates that a common pollutant, OP, affects male sexual behavior possibly by acting like an estrogen.